Silica sols are sedimentation-stable, colloidal solutions of amorphous SiO2 in water or alcohols and other polar solvents. They generally have the flowability of water, and some of the commercial products available to date have high solids concentrations of up to 60% by weight of SiO2.
Silica sols have a variety of uses. For example, they are suitable for use as binders for precision casting, for fibers in the refractories sector and in the preparation of catalysts, as coating agents for films (antiblocking) or silicon steel sheets, in the textile sector for antislip finishes, in the construction sector as additives for air-placed concrete or as binders for fireproofing and heatproofing applications, as polishes for electronics or in the paper sector, for example in paper retention or as an additive in the coating of special papers.
Conventional silica sols are milky opaque through opalescent to colorless and clear, depending on the particle size of the silica particles. The particles of the silica sols have diameters of from 3 nm to 50 nm, preferably from 5 nm to 150 nm. The particles are as a rule spherical and localized and preferably have a negative electrical charge. A framework of siloxane bonds which arises from the linkage of [SiO4] tetrahedra or of polysilicic acids is usually present in the interior of the individual particles. Frequently, SiOH groups are arranged on the surface. Stable silica sols having specific surface areas of from about 30 to 1200 m2/g are preferred for various applications.
The stability of the silica sols is very important. Particularly silica sols which contain very fine SiO2 particles, i.e. silica sols having a large specific surface area, tend to gel formation, so that stabilization is often necessary. Conventional methods for stabilizing silica sols are treatment with alkali metal hydroxides or modification of the surface with aluminum.
U.S. Pat. No. 5,643,414 describes a colloidal finely divided silica sol which has a large BET surface area of greater than 500 m2/g and is stabilized by treatment of the surface with aluminum ions. Furthermore, U.S. Pat. No. 5,603,805 describes an aluminum-stabilized silica sol, which however has a surface area of less than 700 m2/g.
U.S. Pat. No. 6,310,104 B1 describes a finely divided, colloidal borosilicate. According to U.S. Pat. No. 6,310,104 B1, such a colloidal borosilicate is superior to silica sols when used as a paper retention aid.
Common to all these stabilized materials is that they have Si—O—Al linkages or Si—O—B linkages on the surface for stabilization.
U.S. Pat. No. 5,221,497 furthermore discloses silica sols which have so-called structured or partly agglomerated particles. These structured particles consist of small particles which are combined to form chain-like or three-dimensional formations so that the particles have an elongated structure. The individual particles are arranged in each case in a plane so that two-dimensional structures are formed. The presence of an alkali metal oxide is necessary for stabilization. These silica sols are proposed for use in paper retention.
In U.S. Pat. No. 3,630,954, inter alia, a guanidine silicate is used as a raw material for the preparation of fresh sol. According to example 8, a solution of amorphous guanidine silicate is first prepared for this purpose by reacting guanidine hydroxide and silica sol. Said amorphous guanidine silicate is then deionized by means of a dimethylamine-sulfonic acid cation exchanger. In this step, the major part of the guanidinium ions is removed and a dimethylamine-containing silica sol forms, the molar ratio of SiO2 to guanidine oxide being 7.5:1 and the amount of dimethylamine being 1 mol. The surface area, determined by means of base titration according to Sears, is 1500 m2/g. As a result of its preparation, the sol contains large amounts of dimethylamine.
In the preparation of silica sol, in general a fresh sol is first prepared. This is an alkali-free SiO2 solution which is produced, for example, by removing alkali metal cations from a water glass. The resulting fresh sol is very unstable and is therefore immediately stabilized by rendering it alkaline again and by growing on silica sol particles present and by simultaneous thermal treatment in between or afterward. In order to obtain silica sols having a desired content of SiO2, a process for concentrating the aqueous solution can follow. The concentration can be effected, for example, thermally by evaporating down or by ultrafiltration through membranes. Ceramic membranes are suitable for this purpose. Often, the silica sol is stabilized by rendering the solution alkaline to an SiO2:Na2O molar ratio of from 40 to 130:1, heating a part of the solution to 60 to 100° C. for increasing the particle size and then continuously adding the remaining fresh sol solution and allowing it to grow on the particles already present. Simultaneously or thereafter, concentration of the solution to the desired concentration can be carried out by evaporating down. However, a finely divided silica sol rendered alkaline only by means of inorganic bases has the disadvantage that the BET surface area does not remain stable. Such silica sols are therefore generally stabilized with aluminum ions (K. K. Iler, The Chemistry of Silica, Wiley & Sons, New York, 1979, pages 407-410).
The stability to irreversible gel formation to give silica gel, which is based on three-dimensional crosslinking with formation of Si—O—Si bonds between the particles, decreases with increasing silica content, increasing eletrolyte contamination and decreasing particle size. In general, finely divided silica sols, for example those having particle sizes of less than 6 nm, can be brought only to lower solids concentrations of, for example, <20% by weight compared with coarse-particled silica sols having particle sizes greater than 50 nm, in which solids contents up to 60% by weight can be achieved. An increase in the stability of finely divided silica sols is achieved by carrying out a surface modification with aluminum ions, as described in “The Chemistry of Silica by Iler, John Wiley 1978, pages 407-410”.
However, this surface modification is generally carried out after the preparation of the silica sol so that an additional operation is necessary. Moreover, a high aluminum content is undesired in some applications.